Mesozoic cupules and the origin of the angiosperm second integument

The second integument of the angiosperm ovule is unique among seed plants, with developmental genetics that are distinct from those of the inner integument. Understanding how the second integument should be compared to structures in other seed plants is therefore crucial to resolving the long-standing question of the origins of angiosperms. Attention has focused on several extinct plants with recurved cupules that are reminiscent of the anatropous organization of the basic bitegmic ovules of angiosperms, but interpretations have been hampered by inadequate information on the relevant fossils. Here we describe abundant exceptionally well-preserved recurved cupules from a newly discovered silicified peat dating to the Early Cretaceous epoch (around 125.6 million years ago) in Inner Mongolia, China. The new material, combined with re-examination of potentially related fossils, indicates that the recurved cupules of several groups of Mesozoic plants are all fundamentally comparable, and tha...